Polymer blends having a combination of elastic and thermoplastic properties, referred to as xe2x80x9cthermoplastic vulcanizatesxe2x80x9d or xe2x80x9cTPVsxe2x80x9d (also referred to in the past as xe2x80x9cthermoplastic elastomersxe2x80x9d or xe2x80x9cTPEsxe2x80x9d) are made by dynamic vulcanization to provide desired hardness/softness, oil and temperature resistance, oxidation resistance, and processability, inter alia. In thermoplastic elastomers which are elastomeric alloys and not physical blends, the properties depend on the relative amounts of xe2x80x9chardxe2x80x9d and xe2x80x9csoftxe2x80x9d phases provided by each component, and the properties of each component. To be of commercial value, the hard phase is typically provided by a readily available engineering thermoplastic resin, familiarly referred to as a xe2x80x9cplasticxe2x80x9d for brevity. Most commonly the plastic is chosen from polyesters, polyamides and polyolefins which provide a continuous phase of the hard phase in which dispersed domains of the xe2x80x9csoftxe2x80x9d phase of an elastomer are present. Optimizing the elastic recovery of a TPV and confirming the physical nature of its defined morphology, is the subject of this, invention. Confirmation is obtained with both photomicrographs and computer modelling. The photomicrographs are from an electron microscope, preferably a transmission electron microscope (TEM) Of particular interest are relatively xe2x80x9csoftxe2x80x9d blends of a vulcanizable (hereafter xe2x80x9ccurablexe2x80x9d for brevity) rubber having controlled hardness less than about 90 Shore A. Such blends are exceptionally resistant to oil swelling, and to compression set. The term xe2x80x9celastomerxe2x80x9d is used herein to refer to a vulcanized blend of polyolefin and rubber which may be formulated to exhibit varying degrees of elasticity such that a test strip 2.5 cm wide and 25 mm thick may be stretched in the range from about 5% to 100% of its initial length and still return to it; further, such vulcanized elastomer is necessarily thermoplastic and re-processable.
There is a market need for blends of polar engineering thermoplastics containing a dispersed xe2x80x9cpolar rubberxe2x80x9d phase and a continuous xe2x80x9cplasticxe2x80x9d phase, which blends have high elastic recovery. The term xe2x80x9celastic recoveryxe2x80x9d refers to the proportion of recovery after deformation and is quantified as percent recovery after compression. A TPV having a volume fraction of rubber particles greater than about 0.7 may have an elastic recovery in the range from about 50% to 60% at 50% compression; to get a higher elastic recovery one may modify the composition of the particular rubber dispersed in the continuous plastic phase, the ratio of the dispersed and continuous phases, the amounts and composition of the curing agent(s) used, the amount of processing oil, and other ingredients, and other factors, with the expectation that, with enough trial and error, one can make a TPV with an elastic recovery in the range from about 60% to 65%. How these factors influence the morphology of a TPV has been the subject of much study. Very little of this study has been devoted to identifying the key morphological requirement in a TPV which is most likely to provide much higher elastic recovery than one would normally expect of the same TPV produced according to prior art procedures not specifically directed to the formation of the critical morphology.
Elastic recovery is that fraction of a given deformation that behaves elastically; a perfectly elastic material has a recovery of 100% while a perfectly plastic recovery has no elastic recovery. (see Whittington""s Dictionary of Plastics 3rd Ed. 1993 Technomic Publishing). Elastic recovery is an important property of a TPV which is expected to behave like a natural rubber for examples in application where a TPV is used in dynamic applications such as in hoses, and in sealing applications.
To date, a TPV is formulated with specified components including in addition to the rubber and plastic, plasticizers, processing aids and fillers, by melt-blending the ingredients within generally defined processing parameters, until by trial and error, a usable TPV is made. A xe2x80x9cusable TPVxe2x80x9d is one which can be used in a marketable product. In particular, how the components are confined in a mixing and melt-blending means, the rate at which mixing energy is inculcated, the time over which the components are melt-blended, and the conditions under which the TPV is cooled are derived from experience and by trial and error. Though it is likely, with all the work directed to the production of TPVs over the past decade, that TPVs having optimum morphology may have been produced; but if they have been, the morphology produced was accidentally produced. An improvement in elastic recovery was generally sought by varying the curing agent for the rubber, and also the processing oil, processing aid, and filler. No one has recognized, much less identified, the critical morphological feature directly responsible for producing elastic recovery substantially greater than that which is obtained if the critical feature is lacking in a usable TPV.
A usable TPV, contains particles of rubber the majority of which, that is greater than 50% by volume, are in the size range less than about 5 xcexcm, some being as large as 10 xcexcm and others being as small as 0.1 xcexcm or smaller. Particles smaller than 0.1 xcexcm are believed to be portions fractured from larger particles while the TPV is being melt-blended, and this very small size serves to define them as xe2x80x9cvery smallxe2x80x9d particles. A TPV preferred for its superior physical properties and acceptable elastic recovery has relatively large domains of rubber the majority of which are in the size range from about 1-5 xcexcm, preferably 1-3 xcexcm, and this size range serves to define them as xe2x80x9clarge particlesxe2x80x9d. The shape of all particles resembles that of a distorted ellipsoid or elongated ovoid, and this shape is particularly evident in large particles. The remaining rubber particles, in the size range larger than a xe2x80x9cvery smallxe2x80x9d particle and smaller than the mean diameter of the xe2x80x9clarge particlesxe2x80x9d, are defined as xe2x80x9csmall particlesxe2x80x9d or xe2x80x9cmid-range particlesxe2x80x9d which also are generally ellipsoidal in shape. Because of the shape, the xe2x80x9cdiameterxe2x80x9d referred to is the effective diameter, that is, the diameter the particle would have had if it was spherical. The elongated ovoid shape of the particles allows a high packing fraction of rubber particles in a unit volume of TPV, this being a characteristic of a usable TPV. The number of very small particles is of minor consequence in a TPV; the number of small and large particles is not. To date, there has been no clear teaching as to what effect the size of the particles and their distribution has in a TPV particularly with respect to its elastic recovery.
The morphology of various TPVs has been characterized in an article titled Morphology of Elastomeric Alloys by Sabet Abdou-Sabet and Raman P. Patel (Rubber Chem. and Tech., Vol 64, No. 5,pg 769-779, Nov.-Dec. 1991). Several variables affecting the morphology are identified, including the molecular weight of EPDM and PP; the ratio of EPDM to PP; degree of crosslinking; and types of crosslinks; but the effect of the thickness of a ligament, or the volume of continuous plastic phase between adjacent particles was not appreciated. The term xe2x80x9cligamentxe2x80x9d as used herein refers to the material of the continuous plastic phase connecting two adjacently disposed particles, and the xe2x80x9cthickness of a ligamentxe2x80x9d refers to the minimum narrowed distance between two adjacent particles.
The origin of the overall elastomeric-like stress-strain behavior of a TPV including a large percentage of recoverable strain upon unloading is addressed in publications by Kikuchi et al (1992), Kawabata et al (1992) and Soliman et al (1999). In an article titled Origin of Rubber Elasticity in Thermoplastic Elastomers Consisting of Crosslnked Rubber Particles and Ductile Matrix, by Y. Kikuchi, T. Fukui, T. Okada and T. Inoue (Jour. of Appl. Polym. Sci. 50, 261-271 (1992), the strain recovery of a TPE is analyzed using a two-dimensional model for a two-phase system by finite element analysis (FEA). They concluded that at highly deformed states at which almost the whole matrix has yielded to stress concentration, the ligament matrix between rubber inclusions in the stretching direction is locally preserved within an elastic limit and acts as an in situ formed adhesive for connecting the rubber particles. They failed to appreciate that the thickness of the ligament was critically significant and that ligaments are deformed above the elastic limit, rather than preserved below it.
In an article titled Deformation Mechanism and Microstructure of Thermoplastic Elastomer Estimated On the Basis Of Its Mechanical Behavior under Finite Deformation, Sueo Kawabata, S. Kitawaki, et al. (Jour. of Appl. Polym. Sci. 50, 245-259 (1992), presented a model to describe the large deformation mechanism of EPDM/PP and found that oil domains or layers between blocks play an important role in separating the rubber blocks from each other, allowing them to become free elements without friction between them. But Kawabata et al also failed to recognize the critical function of thin ligaments, less than 0.1 xcexcm thick, between adjacent rubber particles, particularly large and small rubber particles.
For simplicity the description relating to the critical thickness of plural ligaments between a small particle and adjacent large particles, or, between large particles themselves, does not take into consideration components other than the rubber particles and the continuous plastic phase in which they are dispersed. One skilled in the art will recognize that such other components are typically dispersed between both phases, the relative amounts in each phase being determined by the particular composition of each phase and that of the other component. The presence of such other components does not noticeably affect the criticality of the thickness of ligaments with respect to their effect on elastic recovery.
It has been discovered that thin ligaments (as defined herein) connecting adjacent particles, and particularly xe2x80x9csmallxe2x80x9d and xe2x80x9clargexe2x80x9d particles of rubber, is the critical determining factor which provides substantially higher elastic recovery than obtained with ligaments thicker than 0.1 xcexcm; the mechanism of deformation and of elastic recovery related to the microstructure and mechanical behavior of a TPV is simulated and confirmed by a micromechanical model of a representative volume element (RVE) in which key structural features, particularly ligament thickness and asymmetry, are systematically varied; tensile properties are not significantly affected by a wide range of ligament thickness; in retrospect, having found what the critical requirement is, parameters for rubber particles of any composition, and for any plastic phase, may be used in the model to predict the elastic recovery of the TPV.
It is therefore a general object of this invention to provide a thermoplastic vulcanizate of an elastomer and a plastic comprising particles of elastomer dispersed in a continuous phase of plastic, such that a majority of particles, and particularly a majority of large particles which are present in a major proportion by volume relative to the small particles, are adjacent at least one small particle critically spaced apart by ligaments, and at least 15% of the ligaments have a thickness less than 10% of the mean diameter of large particles, preferably less than 5% of the xe2x80x9cmean large particle diameterxe2x80x9d, and the remaining ligaments have a thickness less than 50%, preferably from 15% to about 30% of the mean large particle diameter. Preferably the mean large particle diameter is in the range from 1 xcexcm to 3 xcexcm, most preferably about 1 xcexcm; and, the small particle diameter is in the range from 1% to 60% of the mean large particle diameter, preferably from 10% to 40%. Having confirmed the essential requirement for optimum, or near-optimum elastic recovery for a specific TPV-R with a micromechanical model, it now permits one to predict what conditions will generate the thin ligaments in any TPV with any other volume fraction and particle characteristics.
It is a specific object of this invention to provide a TPV having a major proportion by volume of rubber relative to plastic (volume fraction greater than 05) with a morphology in which the distribution of small and large particles is such that a small particle is proximately disposed relative to at least 3 large particles; preferably the number of large particles is numerically smaller than the number of small and very small particles combined.
It is another specific object of this invention to modify a finite element analysis machine program to model a xe2x80x9cfive-particlexe2x80x9d RVE (xe2x80x9c5P-RVExe2x80x9d) which is uniquely adapted to mimic the mechanical behavior of small and large rubber particles dispersed in a continuous plastic phase.